The invention is directed to printheads for ink jet printers and more specifically to improved printhead structures and methods for making the structures.
Ink jet printers continue to be improved as the technology for making the printheads continues to advance. New techniques are constantly being developed to provide low cost, highly reliable printers which approach the speed and quality of laser printers. An added benefit of ink jet printers is that color images can be produced at a fraction of the cost of laser printers with as good or better quality than laser printers. All of the foregoing benefits exhibited by ink jet printers have also increased the competitiveness of suppliers to provide comparable printers in a more cost efficient manner than their competitors.
One area of improvement in the printers is in the print engine or printhead itself. Printheads may be classified in several categories which include thermal printheads and piezoelectric printheads. Thermal printheads eject ink by superheating a component in the ink thereby forming a vapor bubble which forces ink through a nozzle hole onto print media. Piezoelectric printheads operate by forming pressure pulses in a pressurizing chamber containing ink using a piezoelectric film adjacent one wall of the chamber. Activation of the piezoelectric film causes a pulsation of a pressurizing chamber wall thereby forcing ink out of a nozzle hole adjacent the pressurizing chamber.
A piezoelectric ink jet printhead includes a pressurizing chamber substrate and a nozzle substrate bonded to the pressurizing chamber substrate. The pressurizing chamber substrate is typically made from a monocrystalline silicon material having a thickness ranging from about 100 to about 800 microns. An oscillating plate film, lower electrode, piezoelectric film and upper electrode are formed on the silicon substrate opposite the pressurizing chamber side of the substrate. The pressurizing chambers are conventionally formed by a wet chemical etching process by etching into the thickness dimension of the silicon substrate.
Wet chemical etching techniques provide suitable dimensional control for etching of relatively thin semiconductor chips. Methods for wet chemical etching silicon to produce pressurizing chambers are described for example in U.S. Pat. No. 5,265,315 to Hoisington et al. However, wet chemical etching is highly dependent on the thickness of the silicon chip and the concentration of the etchant which results in variations in etch rates and etch tolerances. The resulting etch pattern for wet chemical etching must be at least as wide as the thickness of the wafer. Wet chemical etching is also dependent on the silicon crystal orientation and any misalignment relative to the crystal lattice direction can affect dimensional control. Mask alignment errors and crystal lattice registration errors may result in significant total errors in acceptable product tolerances. Accordingly, wet chemical etching is not practical for relatively thick silicon substrates because the entrance width is equal to the exit width plus the square root of 2 times the substrate thickness. However, it is desirable to use standard silicon wafers which are relatively thick. Obtaining thinner silicon wafers increases the costs of the product due to the non-standard thickness.
Other problems associated with wet chemical etching include, undercutting of the pressurizing chambers, especially when forming deep trench structures. It becomes extremely difficult, if not impossible, to form well defined, sharp and high-aspect ratio trench structures by a wet chemical etching process. In addition, the toxicity of the wet chemical etchant also poses environmental problems and extreme caution must be exercised when handling the wet etchant chemicals. A boron-diffused layer is desirably used as an etch-stop layer for wet chemical etching of the silicon substrate. However, providing a boron-diffused layer requires well controlled diffusion techniques which substantially increases the cost of printhead construction.
Despite their seeming simplicity, printhead devices described above are microscopic marvels containing electrical circuits, ink passageways and a variety of tiny parts assembled with precision to provide a powerful, yet versatile component of the printer. The printhead components must cooperate with an endless variety of ink formulations to provide the desired print properties. Accordingly, it is important to match the printhead components to the ink and the duty cycle demanded by the printer. Slight variations in production quality can have a tremendous influence on the product yield and resulting printer performance.
As advances are made in print quality and speed, a need arises for an increased number of pressurizing chambers and associated nozzle holes which are more closely spaced on the silicon substrates. Decreased spacing between the nozzles and pressurizing chambers requires more reliable manufacturing techniques and manufacturing techniques having lower tolerances. As the complexity of the printheads continues to increase, there is a need for long-life printheads which can be produced in high yield while meeting the more demanding manufacturing tolerances. Thus, there continues to be a need for improved manufacturing processes and techniques which provide improved printhead components.
With regard to the above and other objects the invention provides a method for making piezoelectric printheads for ink jet printers. The method includes applying an insulating layer to a first surface of a silicon wafer having a thickness ranging from about 200 to about 800 microns. A first conducting layer is applied to the insulating layer on the first surface and a piezoelectric layer is applied to the conducting layer. The piezoelectric layer is patterned to provide piezoelectric elements on the first surface of the silicon wafer. A second conducting layer is applied to the piezoelectric layer and is patterned to provide conductors for applying an electric field across each of the piezoelectric elements. A photoresist layer is applied to a second surface of the silicon wafer, and the photoresist layer is imaged and developed to provide pressurizing chamber locations. The silicon wafer is then dry etched through the thickness of the wafer up to the insulating layer on the first surface of the wafer. A nozzle plate containing nozzle holes corresponding to the pressurizing chambers is applied and bonded to the second surface of the silicon wafer.
In another aspect the invention provides a piezoelectric printhead for an ink jet printer. The printhead includes a silicon wafer having a thickness ranging from about 200 to about 800 microns, a first surface and a second surface. The first surface contains an insulating layer, conducting layer, piezoelectric layer and electrical contact layer and the second surface optionally contains a passivation layer. A plurality of pressurizing chambers having substantially vertical walls are dry etched in the silicon wafer through the passivation layer on the second surface up to the insulating layer on the first surface. A nozzle plate containing nozzle holes corresponding to each of the pressurizing chambers is attached to the second surface of the silicon wafer.
An advantage of the invention is that pressurizing chambers and ink vias may be formed in a relatively thick semiconductor silicon chip with substantially consistent tolerances to provide improved ink flow characteristics as compared to printheads made using wet chemical etching techniques. Deep reactive ion etching (DRIE) and inductively coupled plasma (ICP) etching, referred to herein as xe2x80x9cdry etchingxe2x80x9d, provide advantages over wet chemical etching techniques because the etch rate is not dependent on silicon thickness or crystal lattice orientation and thus undercutting of the pressurizing chambers is greatly reduced. Dry etching techniques are also adaptable to producing a larger number of pressurizing chambers which may be more closely spaced together than pressurizing chambers made with conventional wet chemical etching processes. The dry etching techniques of the invention also avoid the use of highly corrosive chemicals for producing high aspect ratio, relatively deep fluid flow structures in silicon wafers.